Active matrix-type liquid crystal display devices have heretofore usually used a TN (twisted nematic) mode, but the TN mode is disadvantageously narrow in viewing angle characteristics. Therefore, other techniques, such as the MVA (multidomain vertical alignment) mode and the IPS (in-plane-switching) mode, are employed for wide-viewing-angle liquid crystal panels.
In the IPS mode, a pair of flat comb-shaped electrodes opposing to each other are provided in a horizontal plane parallel to the display plane direction, and the liquid crystal molecules between opposing electrodes are switched in the horizontal plane. The flat comb-shaped electrodes extremely decrease the aperture ratio of the liquid crystal display device and therefore, a strong backlight is necessary for the IPS-mode liquid crystal display device.
In the MVA mode, the liquid crystal is oriented vertically to the substrate, and the alignment of liquid crystal molecules is defined by slits provided in a transparent electrode (ITO film) and protrusions provided on the opposing substrate. In the MVA-mode liquid crystal display, although a reduction of the substantial aperture ratio due to slits or protrusions is not so large as in the IPS-mode liquid crystal display device using comb-shaped electrodes, the light transmission of the MVA-mode liquid crystal display device is low compared with the TN-mode liquid crystal display device. Therefore, the MVA-mode liquid crystal display device cannot be employed for laptops requiring low power consumption.
In recent MVA-mode liquid crystal display devices, banks (protrusions) and ITO slits are complicatedly arranged so that liquid crystal molecules turn over in four directions upon application of a voltage to attain wide viewing angle and, therefore, the light transmission is decreased. If this arrangement is simplified and the distance between banks or between slits is expanded, the light transmission can be increased. However, if the distance between banks or ITO slits is significantly large, the propagation of liquid crystal molecule tilting takes much time and when a voltage is applied to the device for display, the response of the device is seriously delayed.
In order to solve this problem of delayed response, in the production of an MVA-mode liquid crystal display device, a technique of injecting a polymerizable monomer-containing liquid crystal material between substrates and polymerizing the monomer in the state of a voltage being applied, thereby memorizing the direction to which liquid crystal molecules turn over, has been introduced.
In general, a liquid crystal display device more or less causes a phenomenon of image burn such that, after the same image is continuously displayed for a long period of time, the previous image is persistently viewed, even if the display image is changed. The generation of this image burn phenomenon is an unavoidable problem also in the liquid crystal display device produced by using the above-described technique of injecting a polymerizable monomer-containing liquid crystal material between substrates and polymerizing the monomer in the state of a voltage being applied, thereby memorizing the direction to which liquid crystal molecules turn over. In fact, a technique capable of reducing this phenomenon without fail has, heretofore, not been known.